Currently, most of the military laser powers reported by the public exceed 10⁴ W level, while domestic measurement regulations exceeding 10⁴ W level have not been released. Most laser power meters imported from abroad are used for power testing, but the measurement uncertainty is large, and strict measurement calibration and measurement value traceability cannot be conducted. Hence, to realize accurate laser power measurement, a high-power laser measurement system based on calorimetry is developed.

The high-power laser power measurement device designed in this study (Fig. 1) (hereafter referred to as the self-developed device) adopts a cylindrical cavity as the laser energy absorption cavity. The laser is incident on the reflection cone at the bottom of the cavity and diffused to the inner surface of the absorption cavity through the reflection cone to heat the absorption cavity. The laser power is determined by calculating the water flow and temperature increase. Through an uncertainty evaluation of the measurement parameters (including temperature difference, flow rate, and absorption coefficient of absorption cavity), the traceability of the high-power laser power value is realized.

During the laser power measurement, it is highly important to ensure the accuracy of the temperature measurement. The main factor that affects the accuracy of temperature measurements is heat loss, which primarily includes spontaneous radiation, flow friction, and heat conduction. Stefan?Boltzmann formula is used to calculate the radiation power of the absorption cavity, and the kinetic energy theorem is used to calculate the heat generated by the dynamic friction of the water flow. It is analyzed that the aforementioned two types of heat losses slightly influence the measurement results, and the main source of heat loss is the heat conduction caused by the surface heat dissipation of the absorbing cavity. To reduce the heat dissipation of the absorption chamber, an insulation layer is covered outside the water-cooling chamber layer to reduce the heat dissipation. The steady-state heat conduction formula of the cylinder wall is used to calculate the heat dissipation on the surface of the absorption cavity, which reduces the influence of heat loss to less than 0.1% and ensures that the thermal insulation performance of the system meets the requirements of the laser measurement.

To test the upper limit of the laser power measurement by the absorption cavity, the thermodynamic simulation of the cavity is conducted with an extreme ring heating model (Fig. 2). When the laser power is 40 kW and water flow is 0.33 L/s, the maximum temperature of the absorption cavity is 477.97 ℃, which is excessively lower than the melting point of the red copper. Therefore, the laser power measured by the self-developed device exceeds 40 kW.

During the laser power measurement, it is determined that the temperature difference measurement exhibits certain fluctuations. To eliminate temperature field fluctuations, a temperature homogenization device is installed after the water outlet of the self-developed device. The comparison results of temperature measurement before and after treatment are shown in Fig. 3, and the temperature fluctuation is within 0.01 ℃.

Under the same laser power, adjusting the water flow rate can control the temperature difference between the inlet and outlet. As a larger temperature difference results in a smaller relative measurement uncertainty, the measurement uncertainty of the temperature difference can be reduced by controlling the water flow rate. The water flow rate is adjusted to 0.230 L/s and 0.029 L/s, respectively, and the temperature difference and power of the fiber laser with a wavelength of 1080 nm and spot size of 3 cm are measured at 1.4 kW (Figs. 6 and 7). The test results show that the method for adjusting the water flow to modify the temperature difference is effective in reducing measurement uncertainty.

Based on the seven aspects of repeatability (Table 1), nonlinearity (Fig. 5), surface uniformity (Fig. 8), temperature difference of the measurement device before and after laser irradiation, flow measurement, temperature difference measurement, absorption coefficient measurement of the absorption cavity, and the uncertainty introduced by the laser measurement, the uncertainties are calculated and traced. The introduced uncertainties are 0.06%, 0.51%, 1.0%, 0.15%, 0.055%, 0.43%, and 0.55% (Table 2). The final calculated extended uncertainty of the laser power measurement is 2.8% (coverage factor k=2). The measurement comparison between the self-developed device and the optical pressure power meter shows that the normalized deviation value of the measurement comparison is less than 1 (Table 3), indicating that the measurement results based on calorimetry are accurate and reliable. This suggest that it is initially possible to use the device as a measurement standard, laying a foundation for the subsequent establishment of high-power laser measurement standards.